The present invention relates to a control device for a zoom lens system of a type in which the amount of movement of a movable lens group (there being at least one such group within the system) along the optical axis for focusing is varied depending on the focal length of the entire optical system.
Throughout this specification, the term "focusing" refers to the focusing of light from an object to be photographed on a predetermined and definite image plane. The term "zooming" refers to a variance in the focal length of the optical system without changing the focusing condition to thereby only vary the magnification of the image of the object on the image plane.
In zoom lens systems, various focusing methods are known in which the front lens or front lens group nearest to the object to be photographed is moved along the optical axis. In the zoom lens system of such types, hereinafter referred to as the first type, the amount of movement of the front lens or the front lens group for focusing is independent of the focal length of the entire optical system, i.e. independent of the zooming position of the optical system.
The zoom lens system of this first type has the advantage that its control device can be of a relatively simple construction. This zoom lens system, however, has several disadvantages such as: the distortion of the image increases according to the increase of movement of the front lens or the front lens group along the optical axis toward the object in order to focus at a near object; and since the aperture of the front lens or front lens group is required to be large in order to prevent a fall off of luminosity of the image at the corner of the picture image when the system is zoomed to a wide-angle view, the zoom lens system is apt to become bulky and heavy and accordingly the zoom lens system becomes expensive and its portability deteriorates.
A different method of focusing is also known in which the focusing lens group to be moved along the optical axis for focusing is not restricted to a front lens or a front lens group, and in which the amount of movement for focusing the lens group is not constant but depends on the focal length of the entire lens system. Hereinafter, this type of zoom lens system will be referred to as the second type.
The zoom lens system of the second type avoids the disadvantages which arise with the first type system. The zoom lens system of this second type, however, does have the disadvantage that as the amount of movement of the movable lens group along the optical axis for focusing on the same object varies depending on the zoomed position of the optical system, it is difficult to perform an exact focusing operation at each zoomed position with a single focusing mechanism such as p rovided in the system of the first type. This particular disadvantage has therefore prevented widespread utilization of the zoom lens system of this type.
A control device has therefore been devised for use in the second type of zoom lens system as now set forth in U.S. Pat. No. 3,850,507. The system disclosed therein includes a number of lens barrels and helicoids interconnecting these barrels. The present invention is an improvement over the varifocal lens assembly of that patent.